Exhaust gas recirculation (EGR), which recirculates a portion of exhaust gas back to an intake side, is widely used for an automotive internal combustion engine. Performing EGR makes it possible not only to reduce NOx in the exhaust gas, but also to provide improved fuel efficiency.
The above effects produced by EGR can be enhanced by introducing an increased amount of EGR gas into a cylinder, that is, by increasing an in-cylinder EGR rate. Meanwhile, increasing the in-cylinder EGR rate is likely to cause the EGR rate to vary from one cylinder to another and degrade the quality of combustion. To avoid such a situation, it is necessary, before anything else, to accurately determine the in-cylinder EGR rate. Thereafter, exercising proper engine control in accordance with the EGR rate or accurately controlling the EGR rate are required.
However, the in-cylinder EGR rate cannot be directly measured. There is no alternative but to indirectly determine the in-cylinder EGR rate in accordance with certain information. A heat release rate and a combustion period have been conventionally used as such information. It is known that the in-cylinder EGR rate is closely related to the heat release rate and combustion period. Although the heat release rate and combustion period cannot be directly measured, they can be accurately calculated from measured data about the in-cylinder pressure as far as the employed internal combustion engine has an in-cylinder pressure sensor.
Inventions disclosed in JP-A-1997-151806, Japanese Patent No. 3158339, and JP-A-2006-336661 relate to EGR and focus attention on the heat release rate and combustion period. The invention disclosed in JP-A-1997-151806 calculates a decrease in an EGR amount in accordance with a heat release rate difference between an EGR-on state and an EGR-off state. The invention disclosed in Japanese Patent No. 3158339 calculates the decrease in the EGR amount in accordance with a combustion period difference between the EGR-on state and the EGR-off state. The invention disclosed in JP-A-2006-336661 calculates a combustion pressure characteristic value for the combustion period from the in-cylinder pressure and provides feedback control of the opening of an EGR valve so as to optimize the combustion pressure characteristic value.
However, the method of calculating the EGR rate from the heat release rate and combustion period has turned out to be impractical from the viewpoint of accuracy. The reason is that the relationship between the in-cylinder EGR rate, the heat release rate, and the combustion period significantly varies with the operating status of the internal combustion engine. To accurately determine the EGR rate from the heat release rate and combustion period, it is necessary to prepare a relevant relational map for each operating state. It means that an enormous amount of map data needs to be prepared. Thus, the adaptation of maps entails heavy workload. In addition, a large memory is required for implementing the maps in a control device.